Effect of Oxide Additions on the Polymorphism of Tantalum Pentoxide: III. "Stabilization" of the Low Temperature...

The "low temperature structure type" of Ta₂O₅ has been found to occur in two distinct forms with the lowest temperature form having a unit cell 14 times the subcell and an intermediate temperature form with a unit cell 11 times the subcell. The two types form intermediate partially ordered mixtures which are apparently in thermal equilibrium at various temperatures between ~ 1000 and 1350 °C. The addition of MoO₃, WO₃, SiO₂, GeO₂, ZrO₂, TiO₂, B₂O₃ and Al₂O₃ each affect the multiplicity of the true unit cell in different ways. WO₃, SiO₂, GeO₂, B₂O₃, and Al₂O₃ form phases structurally similar to "low-Ta₂O₅" which are stable up to the solidus temperatures of the corresponding systems.